From the Spring 2009 issue of Cold Facts (Volume 25, Number 2):
Thanks to a joint project by the US Navy and a number of industry partners, high temperature superconducting (HTS) technology is now at the heart of an advanced degaussing system aboard the USS Higgins at the naval station in San Diego. The HTS degaussing system has already completed initial electrical tests and will undergo a series of sea trials over the next two years [1].
Ships are mainly constructed of steel, causing them to disturb the Earth’s magnetic field. This makes them easily detected by magnetically activated mines. Degaussing is the process of making a (steel) ship’s hull nonmagnetic by producing an opposing magnetic field. Installing degaussing systems to mask a ship’s magnetic signature makes the ship virtually undetectable by magnetic mines, which greatly increases a ship’s survivability.
This is done using a system of electrical cables that runs around the circumference of the ship’s hull, from bow to stern and port to starboard. These magnetic silencing systems have been on board Navy ships since WWII, and have proven a necessity. Since 1950, more US Navy ships have been lost to mines than to missiles, torpedoes or bombs. For example, the USS Princeton and the USS Tripoli suffered more than $167 million in damage in the first Gulf War, a fortune next to the enemy investment of just $15 thousand for the sea mines. [2]
Advanced Degaussing Systems (ADS) are the systems of choice for modern naval construction. While the copper-based systems currently on these classes of ships improve on magnetic signature reduction, they require the use of heavy, bulky equipment that adds weight and maintenance costs.
Demonstrations conducted so far have shown that installation of an HTS degaussing system, which uses a single cable made from HTS wire, cuts down both the overall system weight and installation costs for these classes of ships. In fact, the reduction in weight could be as much as 50%-80%. Even more notable is the reduction in the total installed cable lengths—as much as 90%. Since the HTS wire can be operated at current densities up to 200 times that of copper-based wire systems, not as much wire is necessary, cutting back on the bulk of previous systems. There is also the added benefit of a reduced total cost of ownership compared with systems currently in use [3].
Background and Demonstrations
Brian Fitzpatrick, Project Engineer at the Naval Surface Warfare Center (NSWC) in Philadelphia, is a key member in a team that conducted a feasibility study starting in fiscal year 2004 on the option to replace the copper conducting cabling used in current degaussing systems with HTS cable. Fitzpatrick is the team leader for the Applied Superconductivity and Cryogenics Group at NAVSEA at the NSWC, and his team focuses their research efforts on cryogenic refrigeration systems and superconductivity applications for Navy ships.
The results from that feasibility study were presented at ASNE Day 2005, and an excerpt from the paper reads, “Continued maturity in the High Temperature Superconducting industry and demonstration projects, with both industry and government funding, should allow for continued improvements and cost reductions.” [2]
In an effort to further investigate the benefits of an HTS degaussing system, the Naval Surface Warfare Center Carderock Division (NSWCCD) began demonstrations to verify the performance of the HTS coil [4]. According to the ONR, industry partners for the project were Nexans Deutschland GmbH (represented by El-Tech Technology, Inc.), PHPK Technologies, American Superconductor Corporation, Cryomagnetics and Cryomech [3]. When designing the demonstrations, the Navy chose to utilize Commercial Off the Shelf (COTS) components that would not need as much customization.
“Employing superconducting technology aboard ships presents some challenges,” Fitzpatrick was quoted as saying in Superconductor Week, April 2008. “HTS cables need to be kept at 77K or lower.”[5]
While it had never been demonstrated in a power cable, the decision was made to use a gaseous cryogen for the cable’s cooling system. Safety concerns arise with the use of a liquid cryogen onboard a Navy ship, since asphyxiation and/or “cold burns” could result if there was a system breach. Another reason for choosing gaseous helium was that, unlike liquid nitrogen, which under ambient pressure can only reach 63K before it solidifies, gaseous helium can reach the ideal temperature for operation, 55K [6].
The first demonstration focused on ensuring that a 50-meter cable section could be cooled and maintained at a temperature below 50K. A demonstrator was constructed that consisted of four cryostat sections, each from a different commercial vendor. After some preliminary testing, the decision was made to continue testing on the cryostat section supplied by El-Tech Technology and manufactured by Nexans Deutschland GmbH [6].
For the degaussing system, the Nexans “Cryoflex” was chosen. This flexible cryostat is a vacuum jacketed pipe made of corrugated 316L stainless steel, which is then terminated with a Johnston (bayonet) coupling on each end. Each bayonet of the cryostat was then connected to a junction box designed and built by PHPK Technologies.
The PHPK junction box serves four purposes in the degaussing system. First, it allows for a superconducting splice to be made so the HTS wire forms a continuous current “loop.” Secondly, it provides connection feedthroughs for power to the HTS wire and required monitoring instrumentation. Next, it connects the helium supply and return lines from the refrigeration box to keep the HTS junction at 55K. Finally, it provides a vacuum annulus around all these components to insulate and keep them at cryogenic temperature. Due to the pressures involved, the junction box was designed to meet the intent of the ASME BPV code Section VIII, Division 1, after the splice joint was completed.
The refrigeration box was built by American Superconductor Corporation and consisted of a Cryomech AL330 cryocooler, a Stirling A200 helium circulation fan, heat exchanger, ruthenium oxide temperature sensors and supporting piping [6]. Cryomagnetics, Inc. provided a customized 4G superconducting magnet power supply to energize the cable. American Superconductor also provided the HTS cable, which is made of AMSC’s superconducting wire. In March 2006, AMSC announced the successful operation of their own HTS coil, using wire based on a proprietary cabling technique that allows for a degree of flexibility for the wires and also meets requirements for use in naval ships [7].
The second demonstration by Fitzpatrick and his team improved on some system designs in order to make the transition to a Littoral Combat Ship (LCS). Also, to reduce the refrigeration costs for this design, a single refrigeration system was used to cool the multiple axis degaussing setup. Building and testing of this system were successful, with results similar to the first demonstration [4].
Continuing Work on the HTS Degaussing System
Current work being done on the project focuses on developing a quick disconnect, which would allow two ends of HTS cable to be connected and disconnected rapidly, increasing safety for the ship’s crew and reducing installation time and cost. One challenge that needs to be overcome involves the fact that within the same connector, both a fluid and an electrical connection need to be made and kept at a cryogenic temperature.
A possible solution is being developed and designed by Tai-Yang Research Company (TYRC) in Tallahassee FL, and an alternative connector design is being developed and demonstrated by Creare Inc., a research and development company in Hanover NH. Both companies have been awarded Phase II Small Business Innovative Research (SBIR) contracts for this work, which includes prototyping and field testing [4]. The ultimate goal of Phase III to follow will be commercialization and scale-up for eventual integration into the new ship platforms [5].
The ASNE Day 2007 paper presented by Fitzpatrick concludes that “the technical foundation and understanding of the components and technologies that would be necessary for the implementation of a HTS degaussing system have been successfully determined, demonstrated and validated.” [4] The successful integration of the HTS degaussing system onboard a Navy ship would demonstrate the potential for HTS technology in other naval applications. While that is an eventual goal, the current focus in the years ahead is continued development on the system to make it functional in a shipboard environment.
Contributing Companies’ Contact Information
• For further information on Nexans’ Cryoflex contact Judith Lievesley, El-Tech Technology, Inc., jlievesley@cableconsultantscorp.com, 914/320-9154.
• Contact Cryomech, cryosales@cryomech.com, 315/455-2555.
• For further information about Cryomagnetics’ model 4G power supply, contact Brian Pollard, bpollard@cryomagnetics.com, 865/482-9551, ext 111.
• For more information on custom engineering and fabrication of cryogenic and high vacuum systems contact Tim Savely, PHPK Technologies, tsavely@phpk.com, 614/486-4750.
References:
1.“Superconductors vs. Sea Mines.” Hutchinson, Harry. Mechanical Engineering, September 2008, pg. 13.
2. “High Temperature Superconductor Degaussing System Technology Development.” Fitzpatrick, B., Fikse, T., Robinson, M., 2005, pg. 1-10.
3. “Office of Naval Research Funds First Demonstration of Innovative Superconducting Degaussing System.” Office of Naval Research Press Release, July 2008.
4. “High Temperature Superconductor (HTS) Degaussing System Demonstration and Development.” Fitzpatrick, B.K., Golda, E.M., Kephart, J.T., 2007, pg. 1-8.
5. “Tai-Yang Receives $450,000 to Develop HTS Degaussing Cable Components for US Navy Warships.” Superconductor Week, April 2008, Vol.22, No.5., pg. 1, 6-7.
6. “Characterization of Gaseous Helium Flow Cryogen in a Flexible Cryostat for Naval Applications of High Temperature Superconductor.” Fitzpatrick, B.K., Golda, E.M., Kephart, J.T., August 2006, pg. 1-4.
7. “High Temperature Superconductor Degaussing Coil System.” AMSC Degaussing Coil Data Sheet, retrieved January 15, 2009. From American Superconductor website,
http://www.amsc.com/products/htswire/documents/WFS_DGCOIL_0308_A4_FNL.pdf.
